1. Field of the Invention
This invention relates in general to electrical inductive apparatus, such as transformers, and more particularly to vapor-cooled inductive apparatus.
2. Description of the Prior Art
The combination of gas/vapor medium has proven to be a viable alternative to oil as a dielectric cooling medium to be used in transformers, as well as other electrical apparatus. The limiting factor regarding widespread use has been of an economic nature, i.e., oil is but a fraction of the cost of known vapor alternatives.
A recent advance in the transformer industry that has helped reduce the amount of expensive liquid dielectric necessary for a gas/vapor transformer has been the development of powder coated insulated wire. The development of this insulation technique has enabled the insulation requirements of winding conductors to be reduced to several mils thickness thereby allowing reduction in sizes of the windings and corresponding size reduction of the transformer and required vapor cooling liquid dielectric medium. However, this method of insulation presents a problem because the highly uniform surface covering, normally a desirable by-product of the new insulation technique, does not allow the liquid dielectric to pass between adjacent turns of a winding formed of wire so insulated.
Prior art techniques of providing passages with suitable spacers between the turns of the windings, used in oil filled transformers, are not suitable in vapor-cooled transformers using powder coated insulation for several reasons. First, the difference between the dielectric constants of a gas or vapor media and conventional oil barriers greatly changes the stress grading so that oil structures cannot be effectively used. Second, the passages provided by solid spacers require a larger radial build on the winding, therefore requiring a larger amount of expensive vaporizable liquid dielectric and an increased size in the transformer itself. Third, inserting spacers between the turns of the winding reduces the strength of the winding to withstand short circuit forces.
Accordingly, it would be desirable to have a powder coated insulated coil with integral passages to provide adequate paths for the liquid dielectric to disperse over and flow between the surfaces of an induction winding without the use of solid spacers.